1. Field of the Invention
The invention relates to an automatic calibration circuit, and in particular to an automatic calibration circuit for eliminating an offset voltage of an operational amplifier.
2. Description of the Related Art
Usually, an offset voltage with several millivolts exists in an operational amplifier due to errors during circuit layout and manufacturing process. Basically, an operational amplifier includes a pair of differential transistors. Since it is difficult to manufacture two identical transistors, an offset voltage is caused.
When it is necessary to convert a small input voltage, such as several millivolts, up to about one hundred microvolts or several hundred microvolts into a digital signal, a differential amplifier must amplify the small input voltage in advance. For example, using an analog-to-digital converter for 9-bit conversion with a least significant bit (LSB) equal to 4 mV, when the variation of temperature to voltage of a transistor is dV/dT=-2 mV, the input voltage is required to be amplified if the resolution of temperature is only 2.degree. C. while a sensitivity of 0.1.degree. C. is required for the converter. In this case, the amplifier used to amplify input voltage should have a gain of 20 to meet the resolution of 0.1.degree. C.
However, the amplifier amplifies not only the input voltage, but also its own offset voltage when used. Moreover, if the gain of the amplifier is much larger, the offset voltage is further amplified to a very large voltage value, resulting in an output distortion. For example, when the amplifier has an offset voltage of 5 mA and a gain of 10 (i.e., the input voltage can be amplified 10 times), the output voltage of the amplifier is V.sub.o =10(V.sub.i +5)=10V.sub.i +50, wherein V.sub.i represents a differential input voltage. Obviously, the offset voltage of 5 mV is amplified to 50 mV. Additionally, when the output voltage V.sub.o is input to an analog-to-digital converter, a conversion error on the LSB is caused to create a distortion.
For this reason, the prior converter is limited only to less than a 9-bit conversion. If it is necessary to use an analog-to-digital converter for more than a 9-bit conversion, the prior amplifier formed using a single poly gate structure fails to achieve this purpose. In other words, it is necessary to adopt a double poly gate structure or a no-overlap switch capacitor to achieve this purpose. However, these solutions increase IC manufacturing costs, complexity of circuit layout and difficulty in circuit designs.